Automated wire bonders are employed to make electrical connections between input/output (I/O) pads of semiconductor chips and conductive leads on substrates or lead frames. Most often, each connection is made by way of a very thin gold wire, fed from a spool, through a series of control mechanisms to a capillary where by a combination of heat and ultrasonic energy, a first or ball bond is attached to the chip pad, and a second or stitch bond to the substrate. State-of-the-art bonders operate at very high speeds. For example one interconnection every 250 milliseconds is not an unusual bonding rate.
High-speed bonding equipment is typically programmed by a technician or operator for a selected device lot, the materials are supplied, and the equipment is then expected to operate continuously throughout the required production lot. Highly automated equipment allows one operator or technician to be responsible for several bonding machines. Work stoppages, often signaled by a red light positioned atop the equipment, can occur if the machine is out of material, or if a failure in the bonding cycle is detected. Excessive stoppage events require additional operators, increase cycle time, lower production rates, and generally contribute to an increase in the device cost, and a decrease in productivity.
Many automated wire bonders include sophisticated systems, called bond integrity test systems or “BITS”, intended to detect any failure to make good bonding contact, either to the chip pad, or to the lead. They may also detect an improperly threaded or positioned wire, as required to initiate the next bonding sequence. Frequently work stoppages result from a reported failure of a “lifted ball bond”. An example of a lifted ball bond is illustrated in FIG. 1. This failure is correctly detected by the bond integrity test system, as a failure. It can be seen that the gold ball 15 is not in intimate contact with the bond pad 16 of the semiconductor device, and therefore, an intermittent or incomplete circuit is detected.
In FIG. 2, the ball bond 25 is in intimate contact with the pad 26. However, the bonder BITS frequently reports an erroneous “lifted ball” signal, which in turn requires the attention of an operator to investigate the cause, define the correction, and restart the bonding cycle.
While bond integrity tests do generate a number of erroneous readings, it is necessary that high speed bonders include these automated tests, because failure to detect mis-bonds during the process, and halt the bonding, destroy a large number of chips and substrates if a correction were not made. Without a detection mechanism, there could be a significant cost increase for additional operators and surveillance to minimize the number of devices erroneously bonded.
One common erroneous reading is a “false lifted ball”; a false indication that the ball bond to the chip pad is not properly connected. Typically, the bond integrity test system operates by applying a DC voltage through a current resistor on the electrically isolated bond wire, and providing a means for monitoring the current flow from the device pad to a machine chassis ground. This requires that the wire be insulated or otherwise isolated at the wire feed, and that the fine wire be grounded at the semiconductor or at the work station. Usually, the bonding tool and wire feed are isolated so that the current path from the voltage source is directed through the fine wire to the pad on the device, and to ground. The device under assembly is grounded and a small sense current will flow when the wire has been successfully attached. If the voltage source can be made stable, changes in the observed current are proportional to the impedance of the fine wire, plus the impedance of the device being bonded. However, very small changes will be detected, and a failure reported, whether real or false. The fault is detected in the continuity signature, and a shift in the signature may cause a work stoppage.
Necessarily, bond integrity detection systems are very sensitive, which contributes to the fact that they do cause a number of false work stoppages. Attempts to raise the voltage or current in the fine wire to generate a larger current flow so as to provide larger values for error detection often are harmful to the device under assembly, and therefore, is not an acceptable solution.
An improved BITS system which would eliminate, or minimize the number of erroneous bond failure detections at the first bond would be very beneficial to the industry. It would be particularly helpful if a low cost modification to existing bonders were provided.